FA-92240 / PLC ladder logic scan cycles / Member archive

Open-wire fault forces the value to the bottom of range · case 05

A broken transmitter wire makes the process value jump to its low limit instead of holding.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Scale a 4-20 mA analog channel read once per scan. Raw counts are proportional to 0-20 mA with 20 mA = 31208 counts, so 4 mA = 6242. Below 3.6 mA (raw*200 < 36*31208) the channel reports open wire and above 20.5 mA (raw*40 > 41*31208) over-range; in both cases it returns the last good value (None before any). Otherwise the engineering value in tenths is eng_lo*10 + (raw-6242)*(eng_hi-eng_lo)*10 / 24966 rounded half up, clamped to the configured range (reverse-acting ranges allowed). Return [status, tenths] per scan.

Why this case matters

Ladder programs are executed as repeated scans; each defect here changes what a rung, timer, counter or data-table instruction reports on a particular scan, which is how commissioning and field faults are actually observed.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: scenario 3	`[["ok", 44], ["ok", 170], ["open", 40], ["over", 170], ["ok", 51]]`	`[["ok", 44], ["ok", 170], ["open", 170], ["over", 170], ["ok", 51]]`	Failed

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